1. Field of the Invention
The present invention relates to a communication system that manages a network, by generating network configuration information and network resource reservation information when a terminal comprising the network moves.
2. Description of the Related Art
There are a switching hub and a management-functioning switching hub as examples of switching devices used in a network. These hubs are explained in the followings.
<Switching Hub>
A general switching hub (including a bridge) has a filtering database. The filtering database is sometime called a MAC (physical) address learning table or a filtering table.
When a packet reaches a port of a switching hub, the switching hub correlates the port that has received the packet and a source MAC address of a terminal that has sent the packet. The switching hub stores the correlated port and MAC address in the filtering database. In this way, the switching hub memorizes configuration information that the terminal is connected to a “point” of the port.
The “point” means two cases; one is a case that the terminal is directly connected to the port, and the other is a case that a further switching hub is connected to the port and the terminal is connected to a port of the further switching hub.
A plurality of source MAC addresses may be memorized to one port.
By using the filtering database, when a packet is received, the switching hub looks at a destination MAC address for the packet, and then knows which port the packet should be sent to. The switching hub transmits packets by such a scheme.
<Management-functioning Switching Hub>
A management-functioning switching hub (including a bridge) has a function to return possessing management information, in response to inquiry of an SNMP (Simple Network Management Protocol). The possessing management information includes, for example, a state of a link (link-up/link-down) for the port, a link speed (e.g. 10 Mbps, 100 Mbps), and a filtering database for every port, etc.
Some of the management-functioning switching hubs have a function which is called an SNMP trap. When an event (link-up/link-down of the port, and occurrence of trouble etc.) relating to the switching hub happens, the switching hub can notify a management server of the event by the SNMP trap.
Originally, the SNMP is a protocol for managing a network efficiently. In a prior art communication system, a management device acquires management information for each switching hub, for example, by using the SNMP periodically.
A method of determining a topology of LAN from the filtering database stored in the management-functioning switching hub is described in ‘RFC2108 “Definitions of Managed Objects for IEEE 802.3 Repeater Devices using SMIv2”, p. 75, Section 4, Topology Mapping’.
The prior art communication system presumes that a network possesses a personal computer as a terminal thereof, and basically takes into consideration only a static topology. Therefore, it cannot cope with a possible terminal movement immediately.
In the prior art communication system, timing for updating the filtering database of the switching hub is not relevant to the terminal movement. Therefore, after the terminal movement until a next updating of the filtering database, configuration information of the network, which the management device possesses, does not agree with the actual configuration.
As technology advances, it is obvious that a situation will occur sooner or later, where not only the personal computers but also networking household appliances, able to be connected to a network, are used as such kinds of terminals.
In a network environment represented by the Internet, communication is performed by dividing information to a unit called a packet and transmitting a series of the packet.
Generally, the transmission of a series of the packet is processed by a best effort. This means that all kinds of traffic that may require a real time process or may not are treated equally. Traffic such as audio transmission and video transmission requires the real time process, while traffic such as a file transfer does not require the real time process.
As a result, when the network is crowded with a great amount of traffic such as a lot of file transfer, the audio transmission or the video transmission may be adversely affected, causing a break-off of the audio or a disorder of the video when they are received. Therefore, the transmissions encounter difficulties.
In order to perform the audio transmission and the video transmission, which require the real time process, without difficulties, it is necessary to secure network resources on a communication path. As one of the methods of reserving such network resources, the IETF (Internet Engineering Task Force) has provided a RSVP (Resource Reservation Protocol) as an Internet standard. The contents of the RSVP are described in RFC2205 “Resource ReSerVation Protocol (RSVP)-Version 1 Functional Specification”.
According to the RSVP, before a communication starts, a necessary network resource for the communication is secured in switching devices, which exist on a path to be used for the communication. Thereby, a necessary network resource can be secured for every pieces of traffic, and communication quality can be guaranteed.
An SBM (Subnet Bandwidth Manager) is provided as a protocol to practice an RSVP-based receiving control and network resource management, which operates on an IEEE-802 LAN. The SBM realizes bandwidth reservation in a data link layer or Layer 2 of the OSI Reference Model, by a bandwidth management function called DSBM (Designated SBM). The content of the DSBM is described in RFC2814 “SBM (Subnet Bandwidth Manager): A Protocol for RSVP-based Admission Control over IEEE 802-style networks”.
Next, operation of the RSVP is explained. In the RSVP, bandwidth reservation is made by transmitting and receiving control messages among a switching device, a sending terminal, and a receiving terminal that support the RSVP. The sending terminal transmits a “PATH” message to the receiving terminal. The “PATH” message describes the traffic characteristics of data to be transmitted. The transmitted “PATH” message reaches the receiving terminal via the switching device along the communication path. The receiving terminal in receipt of the “PATH” message transmits a “RESV” message to the sending terminal. The “RESV” message describes a network resource necessary for receiving the data by the receiving terminal itself. Network devices on the path, such as the switching device, reserve their own network resource according to the contents of the “RESV” message. Thereby, the network resource for the communication between the sending terminal and the receiving terminal is reserved. The reserved resource can be continuously held by transmitting the “RESV” message periodically.
Resource reservation represented by the protocols such as the RSVP and the SBM mentioned above, assumes basically a static network configuration. At the time of starting services, network resource reservation is made along a communication path and a reservation message is periodically sent in order to hold the network resource continuously. Therefore, such a resource reservation scheme based on the prior art can not fully cope with a situation where a terminal moves during the communication, thus changing the network configuration. An example of the problem involved in such prior art is described below.
According to the prior art, when a terminal moves and a communication path changes, disagreement occurs between the communication path specified by the reservation information and the actual communication path. While the disagreement continues, a bandwidth for the communication by the sending terminal cannot be guaranteed, and a network resource reservation of other terminals may also be hindered.
FIG. 22 is an explanatory diagram (1), illustrating paths on a network according to the prior art. A terminal 1 and a terminal 2 are connected to a switching device 6; a terminal 3 is connected to a switching device 7; and a terminal 4 is connected to a switching device 5. The switching device 5 is connected to the switching device 6 and the switching device 7. All the switching devices compose a tree-shaped network. Here, transmission bandwidth among the switching devices and between each of the switching devices and a terminal connected to each of the switching devices is assumed to be 10 Mbps, respectively.
It is supposed that network resources are reserved for performing data transmission of a 6-Mbps bandwidth from the terminal 4 to the terminal 3 using a path P1 via the switching device 5 and the switching device 7, and for performing data transmission of the same 6-Mbps bandwidth from the terminal 1 to the terminal 2 using a path P2 via the switching device 6. Therefore, the data transmission between each terminal is smoothly performed in this situation.
It is now supposed that the terminal 2 moves to a place shown in the dotted line from the place shown in the solid line in the direction of an arrow M1, changing the connection from the switching device 6 to the switching device 7. Immediately after the movement, the reservation of the network resource is not made in a new path P3 via the switching device 6, the switching device 5, and the switching device 7. In the situation, if the terminal 1 continues sending packets to the terminal 2, without knowing that the terminal 2 has already moved, conflict may occur on a path between the switching device 5 and the switching device 7. This is because that the bandwidth already used for the data transmission between the terminal 4 and the terminal 3 and the bandwidth required for the new data transmission between the terminal 1 and the terminal 2 via the path P3 add to a sum of 12 Mbps, thus exceeding the transmission bandwidth (10 Mbps) of the path between the switching device 5 and the switching device 7. Therefore, there are some difficulties for each of the data transmission.
Furthermore, the following problems may occur.
FIG. 23 is an explanatory diagram (2), illustrating paths with the same network configuration as shown in FIG. 22. In FIG. 23, a network resource for a 6-Mbps bandwidth data transmission between the terminal 3 and the terminal 2 have been made, using a path P4 from the terminal 3 to the terminal 2 via the switching device 7, the switching device 5, and the switching device 6. It is supposed that the terminal 2 moves from the present place to a new place shown by a dotted line, in the direction of an arrow M2, and changes its connection from the switching device 6 to the switching device 7 and its path from the path P4 to a new path P5. Immediately after the movement, the network resource that the terminal 2 has used along the path P4 is not released yet. At this moment, the terminal 4 tries in vain to reserve a network resource for a 6-Mbps bandwidth data transmission to the terminal 1 along a path P6 via the switching device 5 and the switching device 6. The reservation requested by the terminal 4 can not be fulfilled, although the reservation can be actually made. This is because that the reservation of the network resource between the switching device 5 and the switching device 6 has not yet been released.
Above mentioned problems occur since the resource reservation according to the prior art, which is represented by protocols such as the RSVP and the SBM, cannot instantaneously respond to the terminal movement.
It is impossible to predict actions of general users who use, as terminals, networking appliances that are able to connect to a network. It is considered that such terminals would move more frequently than personal computers that are only used as terminals.
For example, it is plausible to consider that a user extracts a cable connected to a television (an example of the networking appliances) while it is receiving a video, and puts the cable into another port. In that case, the communication path, in which the video data flows, changes. This may adversely affect service for other networking household information appliances.
In a case where a network is composed of a plurality of fixed switching devices and a plurality of terminals (computers, portable video processing devices etc.) that are connected to the switching devices by wireless, it should be presupposed that a connection destination of each of the terminals changes from moment to moment. Therefore, it is expected that the prior art mentioned above is not capable of responding to the situation.
Moreover, under the circumstances of wireless LAN's and mobile devices that have been remarkably spread out recently, it is obvious that changes of communication paths due to the terminal movement may occur frequently. Therefore, a solution for the problem is required.